He is serving a 10-year prison sentence for advocating democratic reform in articles circulated on the internet.

The group is also suing Yahoo! on behalf of Shi Tao, a journalist serving a 10-year sentence for sending an email summarising a Chinese government communiqué on how reporters should handle the 15th anniversary of the 1989 crackdown on the pro-democracy movement.

The suit alleges that these people - and others yet to be identified - were tortured or subjected to inhumane treatment at the hands of the Chinese authorities because of information that Yahoo!, Yahoo! China or Alibaba.com, a Chinese company in which Yahoo! has a minority stake, had passed on to the government.

Shi's case has been taken up by the British human rights group Amnesty International. The group says he is kept under tight control, with family visits requiring special approval from the prison manager, and is not allowed to receive printed matter, including books or newspapers.

Last November, he was awarded the Golden Prize of Freedom by the World Association of Newspapers.

Amnesty has also criticised Yahoo! for providing information to the authorities that led to the arrests and, more generally, the involvement of the company in the practice of government censorship.

In a 40-page defence filed in Oakland, California yesterday, the internet firm argued that US courts were not the place for political grievances against the Chinese government.

"This is a political and diplomatic issue, not a legal one," Kelley Benander, a Yahoo! spokeswoman, told the Los Angeles Times. "The real issue here is the plaintiffs' outrage at the behaviour and laws of the Chinese government. The US court system is not the forum for addressing these political concerns."

Yahoo! does not dispute turning over information in response to Chinese government demands, but argues there was little connection between that information and the arrest, prosecution and conviction of the prisoners.

In its court filings, the company said it "deeply sympathises" with the plaintiffs and their families and does not condone the suppression of their liberties. However, it also argues that the company has no control over Chinese laws or their enforcement.

Human rights groups have criticised other internet companies over their dealings with China.

Google has come under fire for its decision to censor its search services on subjects such as the 1989 Tiananmen Square massacre in order to gain greater access to China's fast-growing market.